Carbon Dioxide Recovery Method Using Cryo-Condensation

ABSTRACT

The present invention relates to a method of capturing carbon dioxide in a fluid comprising at least one compound more volatile than carbon dioxide CO2, for example methane CH4, oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.

The present invention relates to a method of capturing carbon dioxide in a fluid comprising at least one compound more volatile than carbon dioxide CO2, for example methane CH4, oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.

The invention can be notably applied to units producing electricity and/or steam from carbon fuels such as coal, hydrocarbons (natural gas, fuel oil, petrochemical residue, etc), household waste, biomass but can also be applied to gases from refineries, chemical plants, steel-making plants or cement works, to the treatment of natural gas as it leaves production wells. It could also be applied to the flue gases from boilers used to heat buildings or even to the exhaust gases from transport vehicles, and more generally to any industrial process that generates CO2-containing flue gases.

Carbon dioxide is a greenhouse gas. For environmental and/or economic reasons, it is becoming increasingly desirable to reduce or even eliminate discharges of CO2 into the atmosphere by capturing it and then, for example, storing it in appropriate geological layers or by realizing it as an asset in its own right.

A certain number of techniques for capturing carbon dioxide, for example methods based on scrubbing the fluids with solutions of compounds that separate the CO2 by chemical reaction, for example scrubbing using MEA, are known. These methods typically have the following disadvantages:

-   -   high energy consumption (associated with the regeneration of the         compound used to react with the CO2),     -   degradation of the compound that reacts with the carbon dioxide,     -   corrosion due to the compound reacting with the carbon dioxide.

In the field of cryo-condensation, that is to say of cooling until solid CO2 appears, mention may be made of document FR-A-2820052 which discloses a method allowing CO2 to be extracted by anti-sublimation, that is to say by solidification from a gas without passing via the liquid state. The cold required is provided by means of fractionated distillation of refrigerating fluids. This method consumes a great deal of energy.

Document FR-A-2894838 discloses the same type of method, with some of the liquid CO2 produced recirculated. Part of the CO2-lean gas produced is used to condense the water in the process mixture. The cold may be supplied by vaporizing LNG (liquefied natural gas). This synergy reduces the specific energy consumption of the method, although this remains high despite this, and requires an LNG terminal.

It is one object of the present invention to provide an improved method of capturing carbon dioxide from a fluid containing CO2 and at least one compound more volatile than the latter.

The invention relates first of all to a method for producing at least one CO2-lean gas and one or more CO2-rich primary fluids from a process fluid containing CO2 and at least one compound more volatile than CO2 and implementing:

-   -   a) a first cooling of said process fluid by exchange of heat         with no change in state;     -   b) a second cooling of at least part of said process fluid         cooled in step a) so as to obtain at least one solid containing         predominantly CO2 and at least said CO2-lean gas; and     -   c) a step comprising liquefaction of at least part of said solid         and making it possible to obtain said one or more CO2-rich         primary fluids;         said method being characterized in that at least part of said         first cooling performed in step a) is obtained by heating up at         least part of said one or more CO2-rich primary fluids.

The process fluid generally comes from a boiler or any plant that produces flue gases. These flue gases may have undergone various pre-treatments, notably with a view to removing NO_(x) (oxides of nitrogen), dust, SO_(x) (oxides of sulfur) and/or water.

Prior to separation, the process fluid is either monophasic, in gaseous or liquid form, or polyphasic. What is meant by “gaseous” form is “essentially gaseous” form. Specifically, if the process fluid consists of pre-treated flue gases, then it may notably contain dust, solid particles such as soot and/or droplets of liquid.

The process fluid contains CO2 that is to be separated from the other constituents of said fluid by cryo-condensation. These other constituents comprise one or more compounds more volatile than carbon dioxide in terms of condensation, for example methane CH4, oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2. The process fluids generally comprise predominantly nitrogen or predominantly CO or predominantly hydrogen.

In step a) the process fluid is first of all cooled without a change in state. The inventors have demonstrated that this cooling may advantageously take place at least in part by exchange of heat with CO2-rich fluids from the separation process. In addition, it may advantageously take place at least in part by exchange of heat with the CO2-lean gas from the separation process. These cold fluids from the separation process are heated up, while the process fluid is cooled down. This makes it possible to reduce the amount of energy required for the cooling operation.

Step b) consists in solidifying the initially gaseous CO2 by raising the process fluid to a temperature below the triple point for CO2 while the partial pressure of the CO2 in the process fluid is below that of the triple point for CO2. For example, the total pressure of the process fluid is close to atmospheric pressure. This solidification operation is sometimes known as “cryo-condensation” or “anti-sublimation” of the CO2 and, by extension, of the process fluid.

Certain compounds more volatile than CO2 do not solidify and remain in the gaseous state. Together with the non-solidified CO2 these will constitute said CO2-lean gas, that is to say will constitute said gas that comprises less than 50% CO2 by volume and preferably less than 10% CO2 by volume. According to one particular embodiment, said CO2-lean gas contains less than 1% CO2 by volume. According to another particular embodiment, it contains more than 2% thereof. According to another particular embodiment, it contains more than 5% thereof. A solid comprising predominantly CO2, that is to say containing at least 90% by volume if considered in the gaseous state, preferably containing at least 95% by volume, and more preferably still containing at least 99% CO2 by volume, is formed.

This solid may comprise other compounds than CO2. Mention may, for example, be made of other compounds which might also have solidified, or alternatively of bubbles and/or drops of fluid contained within said solid lump. This explains how the solid could potentially consist of not only solid CO2. This “solid” may contain non-solid parts such as fluid inclusions (drops, bubbles, etc).

This solid is then isolated from the compounds that have not solidified after cryo-condensation and recovered. Next, in step c), it is returned to temperature and pressure conditions such that it changes into a fluid, liquid and/or gaseous, state. At least part of said solid may then liquefy. This then gives rise to one or more CO2-rich primary fluids. These fluids are said to be “primary” to distinguish them from treatment fluids which are said to be “secondary”. What is meant by “CO2-rich” is something “comprising predominantly CO2” within the meaning defined hereinabove.

The inventors have demonstrated that it is advantageous to carry out at least part of the first cooling of the process fluid by exchanges of heat with the CO2-rich primary fluids. The advantage is that their cold content is recovered and the need for external cold energy is reduced, this external cold energy generally being provided by one or more refrigerating cycles. This assumes that part of said CO2-rich primary fluids is vaporized and leads to an additional cost in terms of the compression performed with a view to transporting said CO2-rich fluids and/or injecting them into the subsoil.

Depending on circumstances, the method according to the invention may comprise one or more of the following features:

-   -   said step b) takes place at a total pressure such that the         partial pressure of the CO2 contained in said process fluid is         below or equal to that of the triple point for CO2, said total         pressure preferably being close to atmospheric pressure.     -   said step c) occurs at a total pressure above that of the triple         point for CO2, preferably close thereto.     -   step c) comprises a liquefaction obtained by introducing at         least part of said solid into a liquid bath containing         predominantly CO2 and extracting from said liquid bath at least         one CO2-rich primary liquid.     -   said liquid bath is heated by one or more of the following         methods:         -   by exchange of heat with a fluid without said fluid mixing             with said liquid bath;         -   by exchange of heat with a CO2-rich secondary fluid without             said secondary fluid mixing with said liquid bath, said             CO2-rich secondary fluid used to heat said liquid bath             circulating in a closed loop and being heated by exchange of             heat with said process fluid; and/or         -   by introducing and mixing into said liquid bath a CO2-rich             secondary fluid.     -   at least part of said CO2-rich primary fluids is obtained from         said CO2-rich primary liquid by one or more of the following         methods:         -   expansion to produce a fluid at a pressure higher than that             of the triple point for CO2; and/or         -   compression to one or more pressure levels higher than that             of the triple point for CO2. Some of these pressure levels             can be such that the partial pressure of the CO2 is higher             than the critical point pressure of the CO2. Therefore, the             fluid is in a pseudo-liquid state.     -   at least part of said CO2-rich primary fluids comprises a liquid         phase and said heating-up of at least a part of said CO2-rich         primary fluids by exchange of heat with said process fluid         vaporizes at least part of this liquid phase.     -   at least one of said CO2-rich primary fluids remains in the         liquid or pseudo-liquid state during said heating-up of at least         part of said CO2-rich primary fluids by exchange of heat with         said process fluid. The pseudo-liquid state is defined by a         partial pressure of the CO2 higher than the critical pressure         and a temperature comprised between the solidification         temperature and the critical point temperature.     -   at least part of said first cooling performed in step a) is         obtained by exchange of heat with an intermediate fluid that has         exchanged cold with at least part of said one or more CO2-rich         primary fluids.     -   at least part of said CO2-rich primary fluids, after heating-up         by exchange of heat with said process fluid, is compressed to         one or more pressure levels higher than the supercritical         pressure for CO2.

The “liquid bath” mentioned above may be contained in a container. It is generally at a temperature comprised between −50° C. and the temperature of the triple point for CO2 (−56.6° C.), preferably between −55° C. and −56.6° C. It consists predominantly of liquid CO2, resulting from the mounting of said solid obtained by cryo-condensation. The total pressure operating above the liquid bath is higher than or equal to that of the triple point for CO2, preferably close to the latter. A CO2-rich primary liquid is extracted from this bath by any appropriate means.

The solid which is poured into this liquid bath is at a temperature lower than that of the bath, and it is therefore necessary to heat up the bath in order to maintain its temperature and ensure that said solid melts. This heating can be performed in a number of ways, by exchange of heat with one or more other fluids, generally by indirect contact without mixing.

According to one particular embodiment, the exchange of heat may be performed by direct contact by introducing one or more CO2-rich fluids at a temperature higher than that of the bath into the liquid bath. This exchange of heat by direct contact is generally more effective than indirect exchange. A CO2-rich fluid slightly hotter than the liquid bath is then sufficient. This limits irreversibility and improves process efficiency.

The liquid which heats up the liquid bath by indirect exchange is itself cooled. It can therefore be used to cool the process fluid. In this way, the cold energy added to the liquid bath by the solid from the cryo-compensation can be at least partly realized as an asset elsewhere in the process, particularly for the first cooling of the process fluid. The overall efficiency is thus improved.

According to one particular embodiment, several CO2-rich fluids are produced at different pressures. This allows fine adjustment of the quantities of the heat transferred and the energy needed to recompress the CO2-rich primary fluids following the exchange. Low pressures and vaporization of the CO2-rich fluids are favorable to the recovery of cold energy but entail a higher compression cost for these products.

According to another embodiment, the exchange of heat between the CO2-rich fluids and the process fluid may occur via one or more intermediate fluids, that is to say that the CO2-rich fluids will give up part of their cold to said intermediate fluid, which will then give up part of this cold to the process fluid.

The invention also relates to the method applied to industrial flue gases with a view to capturing CO2.

According to one particular embodiment, these flue gases come from a plant producing energy (steam, electricity) and may have undergone pretreatments.

The invention will be better understood on reading the following description and examples, which are non-limiting. They refer to the appended drawings, in which:

FIG. 1 schematically depicts a CO2 capture unit employing a method according to the invention, and

FIG. 2 schematically depicts the use of a method according to the invention in a plant for producing electricity on the basis of coal.

The plant illustrated in FIG. 1 implements the steps described below.

-   -   the fluid 24 consisting of flue gases is compressed in a         compressor 101, notably to compensate for the pressure losses in         the various pieces of equipment in the unit. Let us note that         this compression may also be combined with the compression known         as the draft compression of the boiler that produces the flue         gases. It may also be carried out between other steps of the         method, or downstream of the CO2 separation method;     -   the compressed fluid 30 is injected into a filter 103 to         eliminate particles down to a level of concentration of below 1         mg/m³, preferably of below 100 μg/m³;     -   next, the dust-free fluid 32 is cooled to a temperature close to         0° C., generally of between 0° C. and 10° C., so as to condense         the water vapor it contains. This cooling is carried out in a         tower 105, with water injected at two levels, the cold water 36         and water 34 at a temperature close to ambient temperature. It         is also possible to conceive of indirect contact. The tower 105         may or may not have packings;     -   the fluid 38 is sent to a unit that eliminates residual water         vapor 107, for example using one and/or another of the following         methods:         -   adsorption on fixed beds, fluidized beds and/or rotary             dryer, the adsorbent potentially being activated alumina,             silica gel or a molecular sieve (3A, 4A, 5A, 13X, . . . );         -   condensation in a direct-contact or indirect-contact             exchanger.     -   the dried fluid 40 is then introduced into the exchanger 109         where the fluid is cooled down to a temperature close to, but in         all events higher than, the temperature at which CO2 solidifies.         This temperature can be determined by a person skilled in the         art aware of the pressure and composition of the process fluid         40. This temperature is situated at around about −100° C. if the         CO2 content of the process fluid is of the order of 15% by         volume and for a pressure close to atmospheric pressure.     -   the fluid 42 which has undergone a first cooling 109 is then         introduced into a vessel 111 where it continues to be cooled         down to the temperature that provides the desired level of CO2         capture. Cryo-condensation of at least part of the CO2 contained         in the fluid 42 occurs producing, on the one hand, a CO2-lean         gas 44 and, on the other hand, a solid 62 comprising         predominantly CO2. The gas 44 leaves the vessel 111 at a         temperature of the order of −120° C. This temperature is chosen         as a function of the target level of CO2 capture. At this         temperature, the CO2 content of the gas 44 is of the order of         1.5% by volume, namely a capture level of 90% starting out from         a process fluid containing 15% CO2. There are various         technologies that can be used for this vessel 111:         -   continuous solid cryo-condensation exchanger in which solid             CO2 is produced in the form of carbon dioxide snow, is             extracted, for example, using a screw and pressurized to             introduce it into a bath of liquid CO2 121 in which a             pressure higher than the triple point pressure for CO2             obtains. This pressurization can also be carried out             batchwise in a system of silos. Continuous solid             cryo-condensation may itself be performed in various ways:             -   scraped surface exchanger, the scrapers for example                 being in the form of screws to encourage extraction of                 the solid,             -   fluidized bed exchanger so as to carry the carbon                 dioxide snow along and clean out the tubes using                 particles for example of a density greater than that of                 the carbon dioxide snow,             -   exchanger in which solid is extracted by vibration,                 ultrasound, a pneumatic or thermal effect (intermittent                 heating so as to cause the carbon dioxide snow to fall),             -   accumulation on a surface with periodic “natural” fall                 into a tank,         -   batchwise solid cryo-condensation: in this case, several             exchangers in parallel can be used alternately. They are             then isolated, pressurized to a pressure higher than the             triple point pressure for CO2, so as to liquefy the solid             CO2 and possibly partially vaporize it.     -   the fluid 46 is then heated up in the exchanger 109. As it         leaves, the fluid 48 can also be used notably to regenerate the         unit used for eliminating residual vapor (107) and/or for         producing cold water (115) by evaporation in a direct-contact         tower 115 into which a dry fluid 50 is introduced which then         becomes saturated with water, vaporizing some of it;     -   some of the cold needed for the cryo-condensation performed in         the vessel 111 is supplied by one or more cold sources (75).         Likewise, some of the cold needed for the first cooling 109 is         supplied by one or more cold sources (76);     -   the solid 62 comprising predominantly CO2 is transferred to a         bath 121 of liquid CO2;     -   this bath 121 needs to be heated in order to remain liquid, to         compensate for the addition of cold from the solid 62 (latent         heat of fusion and sensible heat). This can be done in various         ways:         -   by exchange of heat with a hotter fluid 72. The cold energy             from the fluid 74 can be used elsewhere in the method,         -   by direct exchange, for example by tapping a fluid 80 from             the bath 121, heating it in the exchanger 109, and             reinjecting it back into the bath 121;     -   liquid 64 comprising predominantly CO2 is tapped from the bath         121.     -   this liquid is split into three streams. In the example, the         first is obtained by an expansion 65 to 5.5 bar absolute         producing a diphasic, gas-liquid, fluid 66. The second, 68, is         obtained by compression 67, for example to 10 bar. The third,         70, is compressed for example to 55 bar. The 5.5 bar level         provides cold at a temperature close to the triple point         temperature for CO2. The 10 bar level allows the transfer of the         latent heat of vaporization of the fluid 68 at around −40° C.         Finally, at 55 bar, the fluid 70 does not vaporize during the         exchange 109. There is efficient use to be made of the cold         energy contained in the fluid 64 during the exchange 109 while         at the same time limiting the amount of energy required to         produce a purified and compressed stream 5 of CO2;     -   after the exchange 109, the primary fluids 66, 68, 70 are         compressed to a pressure level higher than the critical pressure         for CO2 using the compressors 131, 132, 133.

FIG. 2 depicts a plant for producing the electricity from coal, employing various units 4, 5, 6 and 7 for purifying the flue gases 19.

A primary airflow 15 passes through the unit 3 in which the coal 15 is pulverized and carried along toward the burners of the boiler 1. A secondary airflow 16 is applied directly to the burners in order to provide additional oxygen needed for near-complete combustion of the coal. Feed water 17 is sent to the boiler 1 to produce steam 18 which is expanded in a turbine 8.

The flue gases 19 resulting from the combustion, comprising nitrogen, CO2, water vapor and other impurities, undergo various treatments to remove some of said impurities. The unit 4 removes the NO_(x) for example by catalysis in the presence of ammonia. The unit 5 removes dust, for example using an electrostatic filter, and the unit 6 is a desulfurization system for removing the SO2 and/or SO3. The units 4 and 6 may be superfluous depending on the composition of the product required. The purified flow 24 from the unit 6 (or 5 if 6 is not present) is then sent to a low-temperature cryo-condensation purification unit 7 to produce a relatively pure flow 5 of CO2 and a nitrogen-enriched residual flow 25. This unit 7 is also known as a CO2 capture unit and implements a method according to the invention, as illustrated, for example in FIG. 1.

The main advantages of the invention are therefore:

-   -   a reduction in the power consumption for separating and         compressing the CO2,     -   the possibility of adapting it to suit various operating         constraints, particularly in terms of the pressure at which the         CO2 is to be obtained, the availability and cost of the cold         energy needed for the cryo-condensation separation. 

1-10. (canceled)
 11. A method for producing at least one CO2-lean gas and one or more CO2-rich primary fluids from a process fluid containing CO2 and at least one compound more volatile than CO2, comprising: a) a first cooling of said process fluid by exchange of heat with no change in state; b) a second cooling of at least part of said process fluid cooled in step a) so as to obtain at least one solid containing predominantly CO2 and at least said CO2-lean gas; and c) a step comprising liquefaction of at least part of said solid and making it possible to obtain said one or more CO2-rich primary fluids; wherein at least part of said first cooling performed in step a) is obtained by heating up at least part of said one or more CO2-rich primary fluids.
 12. The method of claim 11, wherein step b) takes place at a total pressure such that the partial pressure of the CO2 contained in said process fluid is below or equal to that of the triple point of CO2, said total pressure preferably being close to atmospheric pressure, and in that said step c) occurs at a total pressure above that of the triple point of CO2, preferably close thereto.
 13. The method of claim 12, wherein step c) comprises a liquefaction obtained by introducing at least part of said solid into a liquid bath containing predominantly CO2 and extracting from said liquid bath at least one CO2-rich primary liquid.
 14. The method of claim 13, wherein said liquid bath is heated by one or more of the following methods: by exchange of heat with a fluid without said fluid mixing with said liquid bath; by exchange of heat with a CO2-rich secondary fluid without said secondary fluid mixing with said liquid bath, said CO2-rich secondary fluid used to heat said liquid bath circulating in a closed loop and being heated by exchange of heat with said process fluid; and/or by introducing and mixing into said liquid bath a CO2-rich secondary fluid.
 15. The method of claim 13, wherein at least part of said CO2-rich primary fluids is obtained from said CO2-rich primary liquid by one or more of the following methods: expansion to produce a fluid at a pressure higher than that of the triple point of CO2; and/or compression to one or more pressure levels higher than that of the triple point for CO2.
 16. The method of claim 11, wherein at least part of said CO2-rich primary fluids comprises a liquid phase and in that said heating-up of at least a part of said CO2-rich primary fluids by exchange of heat with said process fluid vaporizes at least part of this liquid phase.
 17. The method of claim 11, wherein at least one of said CO2-rich primary fluids remains in the liquid or pseudo-liquid state during said heating-up of at least part of said CO2-rich primary fluids by exchange of heat with said process fluid.
 18. The method of claim 11, wherein at least part of said first cooling performed in step a) is obtained by exchange of heat with an intermediate fluid that has exchanged cold with at least part of said one or more CO2-rich primary fluids.
 19. The method of claim 11, wherein at least part of said CO2-rich primary fluids, after heating-up by exchange of heat with said process fluid, is compressed to one or more pressure levels higher than the supercritical pressure for CO2.
 20. The method of claim 11, wherein said method is applied to industrial flue gases with a view to capturing CO2. 